1. Field of the Invention
This invention relates to the field of radio frequency antennas. More particularly to the use of radio frequency antennas as guidewires used in vivo in conjunction with magnetic resonance imaging techniques.
2. Description of Related Art
Magnetic resonance imaging (MRI) is a well known, highly useful technique for imaging matter. It has particular use with imaging the human body or other biological tissue without invasive procedures or exposure to the harmful radiation or chemicals present with x-rays or CT scans. MRI uses changes in the angular momentum orxe2x80x9cspinxe2x80x9d of atomic nuclei of certain elements to show locations of those elements within matter. In an MRI procedure, a subject is usually inserted into an imaging machine that contains a large static magnetic field generally on the order of 0.2 to 4 Tesla although machines with higher strength fields are being developed and used. This static magnetic field tends to cause the vector of the magnetization of the atomic nuclei placed therein to align with the magnetic field. The subject is then exposed to pulses of radio frequency (RF) energy in the form of a second, oscillating, RF magnetic field having a particular frequency referred to in the art as a resonant or Larmor frequency. This frequency is equal to the rate that the spins rotate or precess.
This second field is generally oriented so that its magnetic field is oriented in the transverse plane to that of the static magnetic field and is generally significantly smaller. The second field pulls the net magnetism of the atomic nuclei off the axis of the original magnetic field. As the second magnetic field pulses, it pulls the spins off axis. When it is turned off, the spins xe2x80x9crelaxxe2x80x9d back to their position relative to the initial magnetic field. The rate at which the spins relax is dependent on the molecular level environment. During the relaxation step, the precessing magnetization at the Larmor frequency induces a signal voltage that can be detected by antennas tuned to that frequency. The magnetic resonance signal persists for the time it takes for the spin to relax. Since different tissues have different molecular level environments, the differences in relaxation times provides a mechanism for tissue contrast in MRI.
In order to image the magnetic resonance signal it is necessary to encode the locations of the resonant spins. This is performed by applying pulse of gradient magnetic fields to the main magnetic field in each of the three dimensions. By creating this field, the location of resonant nuclei can be determined because the nuclei will resonate at a different Larmor frequency since the magnetic field they experience differs from their neighbors. The magnetic resonance (MR) image is a representation of the magnetic resonance signal on a display in two or three dimensions. This display usually comprises slices taken on an axis of interest in the subject, or slices in any dimension or combination of dimensions, three-dimensional renderings including computer generated three-dimensional xe2x80x9cblow-upsxe2x80x9d of two-dimensional slices, or any combination of the previous, but can comprise any display known to the art.
MR signals are very weak and therefore the antenna""s ability to detect them depends on both its size and its proximity to the source of those signals. In order to improve the signal of an MRI, the antenna may be placed near or inside the subject to be imaged. Such improvements can enable valuable increases in resolution sensitivity and reduction of scan time. It may be desirable to have evidence of the MRI antenna itself on the MRI to allow the individual inserting the MRI antenna to direct where it is going and to maneuver it with aid from the MR image. Such a benefit could be useful in medical procedures where MRI is used simultaneously to track the position of an intraluminal device and to evaluate the structures surrounding the lumen. For example, an intravascular catheter could be directed through a vessel using MRI to reach a targeted area of the vessel, and the MRI apparatus could further be used to delineate the intravascular anatomy or nearby tissue to determine whether a particular therapeutic intervention would be required. Using MRI to guide the catheter and using MRI further to map out the relevant anatomy could complement conventional angiographic imaging technology within an interventional radiology or cardiology or minimally invasive imaging suite. Once the catheter is directed to the desired anatomic target under MR guidance, and once the topography or other relevant anatomy of the target lesion is depicted using MRI, the clinician can make decisions about what type of intervention would be indicated, if any, and where the intervention should be delivered.
Many conventional vascular interventional procedures use X-ray imaging technology in which guidewires and catheters are inserted into a vein or artery and navigated to specific locations in the heart for diagnostic and therapeutic procedures. Conventional X-ray guided vascular interventions, however, suffer from a number of limitations, including: (1) limited anatomical visualization of the body and blood vessels during the examination, (2) limited ability to obtain a cross-sectional view of the target vessel, (3) inability to characterize important pathologic features of atherosclerotic plaques, (4) limited ability to obtain functional information on the state of the related organ, and (5) exposure of the subject to potentially damaging x-ray radiation.
MRI techniques offer the potential to overcome these deficiencies. However, conventional guidewires are not suitable for use in MRI machines since they contain steel or magnetic materials that can cause significant image artifacts in an MRI machine and can cause injury to a patient from unintended motion due to effects of the magnetic fields or induced Ohmic heating. Additionally, guidewires made of non-magnetic materials (e.g., polymers) to cannot easily be visualized by MRI. Even those antennae which have been fabricated for use inside a human body are not useful for many types of interventional procedures. Many of these devices are simply too large to be sufficiently miniaturized to allow the placement of an interventional device simultaneously with the antenna in a small vessel without causing injury to the subject. Furthermore, many of these devices are not useful as guidewires because the antenna cannot accept the range of interventional tools that are widely used in many types of procedures without removal of the guidewire from the subject during tool transition. This includes, but is not limited to, such tools as balloon catheters for dilatation angioplasties, for stent placements, for drug infusions, and for local vessel therapies such as gene therapies; atherotomes and other devices for plaque resection and debulking; stent placement catheters; drug delivery catheters; intraluminal resecting tools; electrophysiologic mapping instruments; lasers and radio frequency and other ablative instruments. Conventional antennas fail in this regard because they have no method for loading these devices after the antenna has been placed in the subject. The tool must instead be preloaded on the antenna, and then inserted into the subject. If a different tool is needed once the antenna has been inserted, the antenna must be entirely removed, the tool switched, and the antenna reinserted into the subject. This repositioning may require that the antenna be redirected to the lesion with the new tool in place, adding an extra, redundant step with the attendant risks of procedural complications. The more inaccessible the lesion, the greater the potential hazards that a second or subsequent positioning of the antenna may entail. In order to use a range of tools, and be useful for procedures requiring loading of multiple tools during the procedure, it is desirable that the antenna therefore be capable of loading multiple different tools after it has been placed in the subject.
To solve the guidewire visualization problem, two approaches have been proposed: passive visualization, and active visualization. With the passive visualization approach, the material of the guidewires is modified so that the catheter appears bright or dark on MR images. Unfortunately, in these techniques data acquisition speed is often limited and the position of the guidewire cannot be visualized very accurately as it depends on the signal-to-noise ratio (SNR) of a second remote detector coil (antenna) which may be sub-optimal. In addition, the modification of the material may result in image artifacts distorting the view of neighboring tissue. In the active visualization techniques, the MRI signal is received by an antenna placed at the end of the guidewire that potentially provides high SNR and spatial resolution in the vicinity of the antenna. These types of probes have also presented problems for clinical applications, since the antennas are often difficult to insert, providing proper shielding from body fluids and tissues has been difficult, and avoiding injury to patients has at times required suboptimally sized probes to be used.
It is therefore desired in the art to produce a probe that contains an antenna suitable to receive and enhance MR images, that antenna providing signal that renders it visible on an MR image and suitable for use as a guidewire.
It is further desired by the art to provide an MRI probe which is constructed of flexible material that has sufficient mechanical properties to be suitable as a guidewire and suitable electrical properties to be an antenna for MRI images rendering it visible on an MR image.
It is further desired by the art to provide an MRI probe which uses multiple different shaped whip antenna designs to allow specific uses under certain circumstances, and which can be used in a clinical environment.
It is further desired by the art to provide an MRI probe that can act as a guidewire to multiple different interventional tools without having to remove the probe from the body to change between tools.
The invention disclosed herein in one embodiment comprises a system, method, and means for providing a flexible MRI probe assembly which is capable of receiving magnetic resonance signals from a subject and for functioning as a guidewire. To act as a guidewire, in one embodiment the MRI probe is small enough to insert into the guidewire lumen of an interventional device as is known to the art.
In a further embodiment of the current invention, the MRI probe is constructed using materials and designs that optimize mechanical properties for steerability, torque transmission and avoidance of antenna whip failure while maintaining desirable electromagnetic properties in magnetic susceptibly and electrical conductivity.
In a further embodiment of the current invention, the MRI probe""s antenna whip is constructed to be flexible and therefore reduce the risk of chamber or vessel perforation.
In a further embodiment, the invention comprises a system, method, or means, whereby a guidewire probe suitable for use in an MRI machine can have multiple interventional tools switched between and guided by the guidewire probe without having to remove the probe from the subject. This is accomplished in one embodiment of the invention by the design and construction of a probe with a practical connection interface between the probe, the tuning/matching circuitry for tuning the antenna whip, and the MRI machine.
In a further embodiment, the invention provides a magnetic resonance antenna assembly no for receiving magnetic resonance signals from a sample and for functioning as a guidewire, comprising a probe shaft including a core of non-magnetic material, a first insulator/dielectric layer for providing insulation, a shielding layer, a second insulator/dielectric layer, and an antenna whip. The core of non-magnetic material may be made of a super-elastic material, such as Nitinol or any other non-magnetic material whether metallic or non-metallic. The non-magnetic core may include a coating of conductive material which could comprise gold, silver, alternating layers of gold and silver or copper or aluminum, for example. A clip-on connector may be further provided for making an electrical connection to a magnetic resonance scanner, the clip-on connector enabling loading and unloading of interventional devices during a procedure without removal of the probe from the subject. The antenna whip may additionally comprise a linear whip, a helical whip, a tapered or a combination whip depending on the desired mechanical and electric properties of the antenna whip.